1. Field of the invention
This invention relates to an air compression hammer drill bit for directional drilling operations. More particularly, this invention relates to an air compression hammer tool for directional drilling operations. The hammer impacts while simultaneously rotates the bit, thereby assuring maximum penetration of the bit in an earthen formation.
2. Description of the prior art
In percussion drilling, the rock cutting mechanism is of an impacting nature rather than shearing. Therefore, the drill bit rotational parameters, e.g. torque and rpm, are not relevant from a rock formation breaking point of view, except for the necessity that the cutting elements of the bit need to be "indexed" to fresh rock formations. In straight hole air drilling, and especially in mining, this need is achieved by rotating the drill string slowly. This is accomplished in conventional hammer bit operations by incorporating longitudinal splines which key the bit body to a cylindrical sleeve at the bottom of the hammer (commonly known as the driver sub). The drill string rotation is then transferred to the hammer bit itself. Experience has proven that the bit optimum rotational speed is approximately 20 rpm for an impact frequency of 1600 bpm (beats per minute). This rotational speed translates to an angular displacement of approximately 4 to 5 degrees per impact of the bit against the rock formation. Another way to express this rotation is the cutters positioned on the outer row of the hammer bit move at the approximate rate of one half the cutter diameter per stroke of the hammer.
An example of a typical hammer bit connected to a rotatable drill string is described in U.S. Pat. No. 4,932,483. The downhole hammer comprises a top sub and a drill bit separated by a tubular housing incorporating a piston chamber therebetween. A feed tube is mounted to the top sub and extends concentrically into the piston chamber. A piston is slidably received within the housing and over the feed tube. Fluid porting is provided in the feed tube and the piston to sequentially admit fluid in a first space between the piston and top sub to drive the piston towards the drill bit support and to a second space between the piston and the drill bit support to drive the piston towards the top sub.
Rotary motion is provided to the hammer assembly and drill bit by the attached drill string powered by a rotary table typically mounted on the rig platform.
The rotation of the drill string in the conventional hammer bit operation described above, takes away the ability to turn, build, or drop angles which are fundamental in directional drilling operations. A method to rotate the hammer bit without rotating the drill string is instrumental in any directional drilling or steerable system. Such a rotation can be accomplished by a motor mechanism positioned above the hammer that induces rotational motion to the bit itself.
The air percussion hammer tool taught in this specification has particular application for use with the technology taught in U.S. Pat. No. Re. 33,751 entitled SYSTEM AND METHOD FOR CONTROLLED DIRECTIONAL DRILLING assigned to the same assignee as the present invention and incorporated hereby by reference. The patent teaches a system and method for controlled directional drilling utilizing a system approach to design the hardware for drilling according to the well plan. The bend angle of a bent housing, connected between the bit and downhole motor, the diameter of a plurality of stabilizers and placement of the stabilizers with respect to the drill bit are selected and predetermined on the basis of the desired well plan. With the use of an MWD, the direction of the progressing borehole is tracked from the surface. Direction changes as required are controlled from the surface simply by controlling rotating of the drillstring. For curved path drilling, only the downhole motor or, the air percussion hammer of the present invention is rotated causing the borehole to travel along the curve determined by the bend angle in the bent housing and the diameter and location of the concentric stabilizers. When straight hole drilling is required, both the downhole motor or air percussion hammer and the entire drill string are rotated, effectively nullifying the effect of the bend angle in the bent housing.